1. Field of the Invention
The present invention relates to a photocatalyst and a process for purifying water by utilizing the function of the photocatalyst.
2. Description of the Related Art
Inhabitant waste waters, irrigation discharge waters and industrial exhaust waters often contain a lot of nitrogenous and phosphorous materials which are provoking eutrophication phenomenon in lakes, marshes, rivers, sea gulfs and bays. The eutrophication causes enhanced growth of plankton, picoplankton, water-bloom, peridinium, and algae, which renders the waters moldy owing to malodorous 2-methylisoborneol given off by a kind of plankton, phormidium or oscillatoria and adversely affects inhabitant environment, especially water for living. The eutrophication promotes production of water-bloom and limnetic red-water coloring waters of lakes, marshes and rivers green or brown as well as production of so-called red-water coloring sea water reddish brown, pink or brown, which impair the scenery, consume oxygen in waters to cause the lack of oxygen therein. The enormous growth of plankton may cause clogging of the gills of fish with the plankton resulting in tremendous damages in marine products. The extraordinary growth of algae may cause clogging of filter screens in water purification facilities, dams and cleaning ponds to interfere with water purifying treatment.
Moreover, inhabitant waste waters, irrigation discharge waters and industrial exhaust waters may contain fungi such as Eumycetes, actinomycetes, for example, mold, bacteria such as coliform bacteria which may be grown in lakes, marshes, rivers, sea gulfs and bays. Microorganisms includes not a few harmful bacteria such as infectious disease-causing bacteria such as typhoid bacillus and dysentery bacillus, corrosion promoting bacteria such as sulfur bacteria, iron bacteria, sulfate-reducing bacteria, slime bacteria, Eumycetes, actinomycetes rendering water malodorous which have led to occurrence of various disasters. Particularly, in nursery regions such as ponds and water containers cultivating animals living in water such as fish, shell-fish, crab, lobster and frog, and plants living in water such as seaweeds and marine algae as well as ornamental ponds and water baths raising fish and the like, there are often caused contamination of water and evolution of malodorous gases owing to fecal matter and deteriorated feed, and growth of fungi and bacteria on the fecal matter and deteriorated feed.
Aside from the foregoing, inhabitant waste waters, irrigation discharge waters and industrial exhaust waters may contain oxygen-demanding substances such as detergents, oils and the like, deleterious substances such as halogenated organic compounds which may be contained in waste water from semiconductor production factories, and pesticides. These deleterious substances may contaminate lakes, marshes, sea gulfs and bays with damages to organisms living therein.
The methods to be employed for killing the algae, fungi and bacteria grown include, for example, treatments with chlorine, ozone, copper sulfate, and ultraviolet radiation. In order to remove the malodorous substances and colored materials generated by the algae, fungi and bacteria, for example, an adsorption method with activated carbons has been employed. Particularly, water purification facilities drawing waters from lakes, marshes and rivers which have been seriously contaminated have expended substantial effort to improve the quality of the waters by introducing a tremendous amount of activated carbon. On the other hand, an attempt has been proposed to effect sterilization or deodorization by making use of high reducing ability of electrons generated in the conduction band and high oxidizing ability of holes generated in the valence band due to the light excitation which may be caused by irradiating photosemiconductor particles such as titanium oxy compounds with light rays having an energy exceeding the forbidden bandgap.
The aforementioned techniques using treatments with chlorine and ozone can reduce the amounts of the algae, fungi and bacteria in an insufficient manner to achieve a desired effect. In addition they may produce problems of requiring an extended period of time for the treatments and residual chemical compounds in the treated waters which may be the used chemicals themselves or compounds derived therefrom. The adsorption technique using activated carbon can not achieve extinction of the algae, fungi and bacteria, though it can reduce odor and color. The aforementioned method using photosemiconductor particles accomplishes the treatment in the system containing dispersion of ultra-fine particles of photosemiconductor in order to enhance the utilization efficiency of the light rays irradiated as well as the photocatalytic function of the photosemiconductor particles. For this reason, the photosemiconductor particles must be separated from the treated water systems and this separation is extremely difficult. Thus the method has not found practical application.
The present inventors have made an intensive research to develop process for purifying water by achieving extinction of harmful organisms such as algae, fungi and bacteria and decomposition of deleterious substances taking account of the photocatalytic function of photosemiconductor particles. As a result, we have found that (1) the use of composites comprising various supports having photosemiconductor particles deposited on their surfaces in order to facilitate the separation of photosemiconductor particles to be used as photocatalyst leads to occurrence of abrasion and fracture of the composites due to collision with one another and contact with the treated water accompanied by release of photosemiconductor particles from the surfaces of the supports and exposure of the faces on which no photosemiconductor particle is deposited, thereby cause a reduction in photocatalytic function of the composites in a short period of time, while the use of inorganic porous particles as alternative supports allows retention of the photocatalytic function of the composites for a prolonged period of time owing to the photosemiconductor particles deposited on the pores and/or voids of the inorganic porous particles; (2) disposition of such photocatalysts in the places capable of coming into contact with the water to be treated and subsequent irradiation of the photocatalysts with light rays containing ultraviolet radiation enable the killing of algae, fungi and bacteria, deodorization, decoloration, and decomposition of deleterious substances to be performed easily and conveniently, resulting in purification of the treated water; (3) moreover, microorganisms having a function of water purification can be immobilized on the inorganic porous particles together with deposition of photosemiconductor particles so that both the photocatalytic function of the photosemiconductor particles and the water purification function of the microorganisms can facilitate the purification of water in a short period of time. The present invention is completed based on these findings and further investigation.